The present invention relates to control valve assemblies comprising a valve body with an inlet-end, an outlet-end, and a passageway therebetween, and a valve trim comprising a valve stem and a valve seat. The valve stem has a plug seat which during operation engages a valve seat forming a fluid-tight seal. More particularly the present invention relates to improved control valve assemblies suitable for use as flow or pressure control valves in particulate-laden or sticky fluid systems.
Control valve assemblies, particularly those with a flow coefficient C.sub.v as defined in the "Masoneilan Handbook for Control Valve Sizing" Sixth Edition (McGraw-Edison, Norwood, Mass.) 1977, less than 0.5, in processes involving particulate-laden fluids such as biological sludges and gases generated by the gasification of fossil fuels, have been known to plug to the extent that flow control is no longer possible causing the process or part of the process to be shut down or the plugged valve assembly to be isolated from the process to enable removal, cleaning, reinstallation, and return to process control function. This is a bothersome, uneconomical and time-consuming exercise that is a problem in such processes including the growing fields of biological waste treatment and coal gasification. To date, the only recourse has been to provide elaborate filtering systems upstream of the valve. This option is not desirable. According to the ISA Handbook of Control Valves, 1976, Second Edition, J. W. Hutchinson, Editor in Chief, Instrument Society of America, Pittsburg, Pa., smooth bore body designs with few dead ends are recommended under these conditions. While eliminating dead ends in the control valve assembly may slow down build up of foreign material, it does not solve the problem of particulates plugging the gap between the inner surface of a tapered valve seat and the outside surface of a tapered plug.